1. Technical Field of the Invention
The present invention relates to a device for measuring the displacement of a swivel of a drilling mast, and, more particularly, to such a device which is intended to manage the position of the tool in the well as well as the bottom dimension of this same well.
2. Description of the Prior Art
Two types of devices exist which are able to be used on a drilling mast so as to rotationally drive a string of pipes. The first, and the more conventional, is the turntable which is mounted on the floor of the drilling mast and comprises a square or hexagonal opening intended to receive a driving pipe of complementary cross-section. The table is set in rotation and thus gives rise to the rotation of the string of pipes.
The other type of device is the motorised swivel, wherein an electric or hydraulic motor, mounted adjacent to the travelling block, is directly connected to the upper end of the string of pipes. The motor is mounted on a carriage which slides on rails arranged vertically on the mast thus allowing the displacement of the carriage whilst absorbing the torque generated during drilling.
Regardless of the type of driving device, the driller always needs to determine the exact position of the elements of the string of pipes, and in particular of the tool arranged in the well so as to manage the drilling better. The conventional manner for determining the position of the tool consisted simply in noting the number of pipes as well as the other elements of the mass-of-pipes and stabilisers packing used and to deduce the total length therefrom. Taking into account the significant number of pipes used, a number reaching several hundred for a deep drilling, the possibility of error was not negligible. However, the consequences of such an error can be very severe if an operation is undertaken in the well at an incorrect depth.
With a motorised swivel, the alternated displacement along the rails is directly linked to the length of the pipes inserted in the drilling well. Thus, careful measurement of the total displacement of the motorised swivel during a manoeuvre or whilst drilling makes it possible to determine, with precision, the position of the tool in the well, a position which is corrected or otherwise for thermal and pressure effects and for the extension effect of the string of pipes under its own weight.